 # Physics 111: Mechanics Lecture 2

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Physics 111: Mechanics Lecture 2
Wenda Cao NJIT Physics Department

Motion along a straight line
Position and displacement Average velocity and average speed Instantaneous velocity and speed Acceleration Constant acceleration: A special case Free fall acceleration September 8, 2008

Motion Everything moves!
Motion is one of the main topics in Physics 111 Simplification: Moving object is a particle or moves like a particle: “point object” Simplest case: Motion along straight line, 1 dimension LAX Newark September 8, 2008

One Dimensional Position x
What is motion? Change of position over time. How can we represent position along a straight line? Position definition: Defines a starting point: origin (x = 0), x relative to origin Direction: positive (right or up), negative (left or down) It depends on time: t = 0 (start clock), x(t=0) does not have to be zero. Position has units of [Length]: meters. x = m x = - 3 m September 8, 2008

Vector and Scalar A vector quantity is characterized by having both a magnitude and a direction. Displacement, Velocity, Acceleration, Force … Denoted in boldface type with an arrow over the top. A scalar quantity has magnitude, but no direction. Distance, Mass, Temperature, Time … For the motion along a straight line, the direction is represented simply by + and – signs. + sign: Right or Up. - sign: Left or Down. 2-D and 3-D motions. September 8, 2008

Quantities in Motion Any motion involves three concepts
Displacement Velocity Acceleration These concepts can be used to study objects in motion. September 8, 2008

Displacement Displacement is a change of position in time.
f stands for final and i stands for initial. It is a vector quantity. It has both magnitude and direction: + or - sign It has units of [length]: meters. x1 (t1) = m x2 (t2) = m Δx = -2.0 m m = -4.5 m x1 (t1) = m x2 (t2) = m Δx = +1.0 m m = +4.0 m September 8, 2008

Distance and Position-time graph
Displacement in space From A to B: Δx = xB – xA = 52 m – 30 m = 22 m From A to C: Δx = xc – xA = 38 m – 30 m = 8 m Distance is the length of a path followed by a particle from A to B: d = |xB – xA| = |52 m – 30 m| = 22 m from A to C: d = |xB – xA|+ |xC – xB| = 22 m + |38 m – 52 m| = 36 m Displacement is not Distance. September 8, 2008

Velocity Velocity is the rate of change of position.
Velocity is a vector quantity. Velocity has both magnitude and direction. Velocity has a unit of [length/time]: meter/second. Definition: Average velocity Average speed Instantaneous velocity September 8, 2008

Average Velocity Average velocity It is slope of line segment.
Dimension: [length/time]. SI unit: m/s. It is a vector. Displacement sets its sign. September 8, 2008

Average Speed Average speed Dimension: [length/time], m/s.
Scalar: No direction involved. Not necessarily close to Vavg: Savg = (6m + 6m)/(3s+3s) = 2 m/s Vavg = (0 m)/(3s+3s) = 0 m/s September 8, 2008

Graphical Interpretation of Velocity
Velocity can be determined from a position-time graph Average velocity equals the slope of the line joining the initial and final positions. It is a vector quantity. An object moving with a constant velocity will have a graph that is a straight line. September 8, 2008

Instantaneous Velocity
Instantaneous means “at some given instant”. The instantaneous velocity indicates what is happening at every point of time. Limiting process: Chords approach the tangent as Δt => 0 Slope measure rate of change of position Instantaneous velocity: It is a vector quantity. Dimension: [Length/time], m/s. It is the slope of the tangent line to x(t). Instantaneous velocity v(t) is a function of time. September 8, 2008

Uniform Velocity Uniform velocity is constant velocity
The instantaneous velocities are always the same, all the instantaneous velocities will also equal the average velocity Begin with then x x(t) t xi xf v v(t) vx t ti tf September 8, 2008

Average Acceleration Changing velocity (non-uniform) means an acceleration is present. Acceleration is the rate of change of velocity. Acceleration is a vector quantity. Acceleration has both magnitude and direction. Acceleration has a unit of [length/time2]: m/s2. Definition: Average acceleration Instantaneous acceleration September 8, 2008

Average Acceleration Average acceleration
Velocity as a function of time When the sign of the velocity and the acceleration are the same (either positive or negative), then the speed is increasing When the sign of the velocity and the acceleration are in the opposite directions, the speed is decreasing Average acceleration is the slope of the line connecting the initial and final velocities on a velocity-time graph September 8, 2008

Instantaneous and Uniform Acceleration
The limit of the average acceleration as the time interval goes to zero When the instantaneous accelerations are always the same, the acceleration will be uniform. The instantaneous acceleration will be equal to the average acceleration Instantaneous acceleration is the slope of the tangent to the curve of the velocity-time graph September 8, 2008

Relationship between Acceleration and Velocity
Velocity and acceleration are in the same direction Acceleration is uniform (blue arrows maintain the same length) Velocity is increasing (red arrows are getting longer) Positive velocity and positive acceleration September 8, 2008

Relationship between Acceleration and Velocity
Uniform velocity (shown by red arrows maintaining the same size) Acceleration equals zero September 8, 2008

Relationship between Acceleration and Velocity
Acceleration and velocity are in opposite directions Acceleration is uniform (blue arrows maintain the same length) Velocity is decreasing (red arrows are getting shorter) Velocity is positive and acceleration is negative September 8, 2008

Kinematic Variables: x, v, a
Position is a function of time: Velocity is the rate of change of position. Acceleration is the rate of change of velocity. Position Velocity Acceleration Graphical relationship between x, v, and a An elevator is initially stationary, then moves upward, and then stops. Plot v and a as a function of time. September 8, 2008

Motion with a Uniform Acceleration
Acceleration is a constant Kinematic Equations September 8, 2008

Notes on the Equations Given initial conditions:
a(t) = constant = a, v(t=0) = v0, x(t=0) = x0 Start with We have Shows velocity as a function of acceleration and time Use when you don’t know and aren’t asked to find the displacement September 8, 2008

Notes on the Equations Given initial conditions:
a(t) = constant = a, v(t=0) = v0, x(t=0) = x0 Start with Since velocity change at a constant rate, we have Gives displacement as a function of velocity and time Use when you don’t know and aren’t asked for the acceleration September 8, 2008

Notes on the Equations Given initial conditions:
a(t) = constant = a, v(t=0) = v0, x(t=0) = x0 Start with We have Gives displacement as a function of time, initial velocity and acceleration Use when you don’t know and aren’t asked to find the final velocity September 8, 2008

Notes on the Equations Given initial conditions:
a(t) = constant = a, v(t=0) = v0, x(t=0) = x0 Start with We have Gives velocity as a function of acceleration and displacement Use when you don’t know and aren’t asked for the time September 8, 2008

Problem-Solving Hints
Read the problem Draw a diagram Choose a coordinate system, label initial and final points, indicate a positive direction for velocities and accelerations Label all quantities, be sure all the units are consistent Convert if necessary Choose the appropriate kinematic equation Solve for the unknowns You may have to solve two equations for two unknowns Check your results Estimate and compare Check units September 8, 2008

Free Fall Acceleration
y Earth gravity provides a constant acceleration. Most important case of constant acceleration. Free-fall acceleration is independent of mass. Magnitude: |a| = g = 9.8 m/s2 Direction: always downward, so ag is negative if define “up” as positive, a = -g = -9.8 m/s2 Try to pick origin so that xi = 0 September 8, 2008

Free Fall Acceleration
x Two important equation: Begin with t0 = 0, v0 = 0, x0 = 0 So, t2 = 2|x|/g same for two balls! Assuming the leaning tower of Pisa is 150 ft high, neglecting air resistance, t = (21500.305/9.8)1/2 = 3.05 s September 8, 2008

Summary This is the simplest type of motion
It lays the groundwork for more complex motion Kinematic variables in one dimension Position x(t) m L Velocity v(t) m/s L/T Acceleration a(t) m/s2 L/T2 All depend on time All are vectors: magnitude and direction vector: Equations for motion with constant acceleration: missing quantities x – x0 v t a v0 September 8, 2008